1. Field of the Invention
The present invention relates to wireless communications and more particularly, a method and apparatus for transmitting channel state information in a wireless communication system.
2. Related Art
The next-generation multimedia wireless communication systems which are recently being actively researched are required to process and transmit various pieces of information, such as video and wireless data as well as the initial voice-centered services. The 4th generation wireless communication systems which are now being developed subsequently to the 3rd generation wireless communication systems are aiming at supporting high-speed data service of downlink 1 Gbps (Gigabits per second) and uplink 500 Mbps (Megabits per second). The object of the wireless communication system is to establish reliable communications between a number of users irrespective of their positions and mobility. However, a wireless channel has abnormal characteristics, such as path loss, noise, a fading phenomenon due to multi-path, inter-symbol interference (ISI), and the Doppler Effect resulting from the mobility of a user equipment. A variety of techniques are being developed in order to overcome the abnormal characteristics of the wireless channel and to increase the reliability of wireless communication.
A carrier aggregation (CA) which supports a plurality of cells may be applied in a 3rd generation partnership project (3GPP) long-term evolution advanced (LTE-A). The CA may be referred to as another name such as a bandwidth aggregation. The CA refers to configuring a broadband by collecting one or more carrier having a bandwidth smaller than the broadband when a wireless communication system tries to support the broadband. The carrier which becomes a subject when collecting one or more carrier may use the bandwidth which is used in the existing system for backward compatibility. For example, in 3GPP LTE, the bandwidths of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz are supported, and in 3GPP LTE-A, the broadband of more than 20 MHz may be configured by using only the bandwidth of the 3GPP LTE system. Furthermore, the broadband may be formed by defining a new bandwidth without using the bandwidth of the conventional system as itself.
A user equipment (UE) may transmit various types of uplink control information. Uplink control information may include a scheduling request (SR), hybrid automatic repeat request (HARQ)-acknowledgement (ACK) signal, and channel state information (CSI) including such as a channel quality indicator (CQI), recoding matrix indicator (PMI), and rank indicator (RI), etc. In general, CSI has a lower priority than an SR or HARQ-ACK signal. Therefore, when simultaneous transmission of CSI and other uplink control information is needed, the CSI may in fact be dropped without being transmitted. Also, in the case of simultaneous transmission of the CSI among a plurality of configured serving cells, transmission of CSI of a specific serving cell may be dropped.
The LTE rel-10 system provides two types of CSI transmission methods. The UE may transmit periodic CSI through a physical uplink control channel (PUCCH) or aperiodic CSI through a physical uplink shared channel (PUSCH). The periodic CSI is transmitted continuously at regular intervals while the aperiodic CSI is transmitted according to a command of a base station.
A method for transmitting periodic CSI in an efficient manner is required.